1. Field of the Invention
This invention relates to a laser recording apparatus for recording a continuous tone image on a photosensitive material by scanning the photosensitive material with an analog light beam which was intensity-modulated on the basis of an image signal and, more particularly, to a laser recording apparatus, which uses a semiconductor laser as a light beam source to permit recording of the image at a high speed and with high accuracy.
2. Description of the Prior Art
Heretofore, light-beam-scanning recording apparatuses have been extensively used, in which an image is recorded on a photosensitive material by scanning the material with a light beam which was deflected by a beam deflector. In such light-beam-scanning recording apparatuses, semiconductor lasers are used as a means for generating a light beam. Semiconductor lasers are superior to gas lasers in that they are small in size and inexpensive, consume less power and are capable of direct modulation through control of the current which drives the laser, hereinafter the drive current.
On the negative side, however, the use of a semiconductor laser for the recording of a continuous tone image is difficult because the intensity of the light output, hereinafter simply called the light output, increases much more rapidly after the drive current crosses into a laser oscillation region than it does when the drive current is in a LED oscillation region as shown in FIG. 2. More specifically, if only the laser oscillation region, in which the light output varies linearly as a function of the drive current, is utilized for the light intensity modulation, it is possible to obtain a dynamic light output range of about two digits at the most. With a dynamic range of such a low order, a highly-graduated continuous tone image cannot be obtained as is well known in the art.
More specifically, in order to permit recording of a highly-graduated tone image with a density scale of, for instance, 10 bits, corresponding to 1024 graduations, it is desired to provide a dynamic light output range of, for instance, 3 digits by permitting intensity modulation of the semiconductor laser light output by the drive current over both the LED and laser oscillation regions shown in FIG. 2. However, when the two regions noted above are covered, the light output is no longer a linear function of the drive current. However, in order to accurately record an image with a highly graduated density scale, two constraints exist. First, the change in toner density (on the photoconductive surface) produced for each density graduation in a continuous tone image must be uniform across the density scale. Second, a fixed level of change in an image signal, i.e. the signal which carries information about a continuous tone image, which image is to be reproduced, must be assigned to a graduation in the toner density scale. To this end, it is desirable to convert the relation between the light output and the image signal, which indirectly controls the drive current of the semiconductor laser, into a linear one, i.e. the relation between the image signal and the drive current should not be linear.
A laser operation control circuit (APC) for creating a linear relation between an image signal and the light output is well-known in the art. Define an emission light level instruction signal for the semiconductor laser as a reference signal derived from the image signal, which image signal carries information about a continuous tone image which is to be reproduced. In the APC circuit the laser beam light intensity, i.e. the light output, is detected, and a feedback signal corresponding to the detected light intensity is fed back to a point where it is added to the light emission level instruction signal, i.e. the reference signal. FIG. 3 shows an example of the APC circuit. The APC circuit will now be described with reference to FIG. 3. An emission light level instruction signal Vref which controls the intensity level of the light emitted from a semiconductor laser 1 is supplied through an adder 2 to a voltage/current conversion amplifier 3. The amplifier 3 supplies a drive current, which is proportional to the signal Vref, to the semiconductor laser 1. The semiconductor laser 1 emits a light beam 4. The emitted light beam 4 travels through a scanning optical system (not shown) to be utilized for the scanning of a photosensitive material. Meanwhile, the intensity of a light beam 5, which is emitted from the semiconductor laser 1 in the opposite direction from that of light beam 4, is detected by, for instance, a pin photodiode 6 disposed for monitoring the intensity of the light put out by the semiconductor laser. The intensity of the light beam 5 that is detected in this way is proportional to the intensity of the light beam 4 utilized for image recording. The intensity of the light beam 5, i.e. the output current from the photodiode 6, which represents the intensity of the light beam 4, is converted by a current/voltage conversion amplifier 7 into a feedback signal (voltage signal) Vpd and then supplied to the adder 2. The adder 2 provides a difference signal Ve representing the difference between the emission light level instruction signal Vref and feedback signal Vpd. The difference signal Ve is converted by the voltage/current conversion amplifier 3 into a current for driving the semiconductor laser 1.
If an ideal linear correction could be obtained in the APC circuit described above, the intensity of the light beam 5 would be proportional to the emission light level instruction signal Vref, that is, the intensity of the light beam (i.e. light put out by the semiconductor laser 1) utilized for image recording would be proportional to the emission light level instruction signal Vref.
When an APC circuit such as that described above is used, however, high speed, accurate recording of a continuous tone image is difficult. This problem will now be described in detail. When recording a continuous tone image, the emission light level instruction signal may suddenly change. The APC circuit must respond even to sudden changes in the emission light level instruction signal if it is to accurately control the semiconductor laser drive current. To permit accurate response to the emission light level instruction signal, however, it is necessary to set an extremely high loop gain in the APC circuit. However, there is an upper limit on the loop gain because, if the loop gain of an APC circuit operating at a high speed is too high, oscillation of the circuit results. For the above reason, it is difficult when using the APC circuit to achieve a high enough response speed for accurate image recording.